The performance potential and flexibility of polarized displays, especially those utilizing the electro-optic properties of liquid crystalline materials, has led to a dramatic growth in the use of these displays for a wide variety of applications. Liquid crystal displays (LCDs) offer the full range from extremely low cost and low power performance (e.g. wristwatch displays) to very high performance and high brightness (e.g. AMLCDs for avionics applications, computer monitors and HDTV projectors). Much of this flexibility comes from the light valve nature of these devices, in that the imaging mechanism is decoupled from the light generation mechanism. While this is a tremendous advantage, it is often necessary to trade performance in certain categories such as luminance capability or light source power consumption in order to maximize image quality or affordability. This reduced optical efficiency can also lead to performance restrictions under high illumination due to heating or fading of the light-absorbing mechanisms commonly used in the displays.
In portable display applications such as backlit laptop computer monitors or other instrument displays, battery life is greatly influenced by the power requirements of the display backlight. Thus, functionality must be compromised to minimize size, weight and cost. Avionics displays and other high performance systems demand high luminance but yet place restrictions on power consumption due to thermal and reliability constraints. Projection displays are subject to extremely high illumination levels, and both heating and reliability must be managed. Head mounted displays utilizing polarized fight valves are particularly sensitive to power requirements, as the temperature of the display and backlight must be maintained at acceptable levels.
Prior art displays suffer from low efficiency, poor luminance uniformity, insufficient luminance and excessive power consumption which generates unacceptably high levels of heat in and around the display. Prior art displays also exhibit a non-optimal environmental range due to dissipation of energy in temperature sensitive components. Backlight assemblies are often excessively large in order to improve the uniformity and efficiency of the system.
Several areas for efficiency improvement are readily identified. Considerable effort has gone into improving the efficiency of the light source (e.g. fluorescent lamps) and optimizing the reflectivity and light distribution of backlight cavities to provide a spatially uniform, high luminance light source behind the display. Pixel aperture ratios are made as high as the particular LCD approach and fabrication method will economically allow. Where color filters are used, these materials have been optimized to provide a compromise between efficiency and color gamut. Reflective color filters have been proposed for returning unused spectral components to a backlight cavity. When allowed by the display requirements, some improvement can also be obtained by constricting the range of illumination angles for the displays via directional techniques.
Even with this prior art optimization, lamp power levels must be undesirably high to achieve the desired luminance. When fluorescent lamps are operated at sufficiently high power levels to provide a high degree of brightness for a cockpit environment, for example, the excess heat generated may damage the display. To avoid such damage, this excess heat must be dissipated. Typically, heat dissipation is accomplished by directing an air stream to impinge upon the components in the display. Unfortunately, the cockpit environment contains dirt and other impurities which are also carried into the display with the impinging air, if such forced air is even available. Presently available LCD displays cannot tolerate the influx of dirt and are soon too dim and dirty to operate effectively.
Another drawback of increasing the power to a fluorescent lamp is that the longevity of the lamp decreases dramatically as ever higher levels of surface luminance are demanded. The result is that aging is accelerated which may cause abrupt failure in short periods of time when operating limitations are exceeded.
Considerable emphasis has also been placed on optimizing the polarizers for these displays. By improving the pass-axis transmittance (approaching the theoretical limit of 50%), the power requirements have been reduced, but the majority of the available light is still absorbed, constraining the efficiency and leading to polarizer reliability issues in high throughput systems as well as potential image quality concerns.
A number of polarization schemes have been proposed for recapturing a portion of the otherwise lost light and reducing heating in projection display systems. These include the use of Brewster angle reflections, thin film polarizers, birefringent crystal polarizers and cholesteric circular polarizers. While somewhat effective, these prior art approaches are very constrained in terms of illumination or viewing angle, with several having significant wavelength dependence as well. Many of these add considerable complexity, size or cost to the projection system, and are impractical on direct view displays. None of these prior art solutions are readily applicable to high performance direct view systems requiring wide viewing angle performance.
Also taught in the prior art (U.S. Pat. No. 4,688,897) is die replacement of the rear pixel electrode in an LCD with a wire grid polarizer for improving the effective resolution of twisted nematic reflective displays, although this reference falls short of applying the reflective polarizing element for polarization conversion and recapture. The advantages which can be gained by the approach, as embodied in the prior art, are rather limited. It allows, in principle, the mirror in a reflective LCD to be placed between the LC material and the substrate, thus allowing the TN mode to be used in reflective mode with a minimum of parallax problems. While this approach has been proposed as a transflective configuration as well, using the wire grid polarizer instead of the partially-silvered mirror or comparable element, the prior art does not provide means for maintaining high contrast over normal lighting configurations for transflective displays. This is because the display contrast in the backlit mode is in the opposite sense of that for ambient lighting. As a result, there will be a sizable range of ambient lighting conditions in which the two sources of light will cancel each other and the display will be unreadable. A further disadvantage of the prior art is that achieving a diffusely reflective polarizer in this manner is not at all straightforward, and hence the reflective mode is most applicable to specular, projection type systems.